VIII. Notching of the Scapula After Reverse Arthroplasty. A Concern?

Classification

Scapular notching has been classified into 5 grades by Sirveaux and Nerot: Grade 0 = No notch; Grade 1 = the defect concerns only the pillar; Grade 2 = contact with the lower screw of the baseplate; Grade 3 = extension over the lower screw; and Grade 4 = extension reaches the central peg (15).

Pathogenesis and Progression

Notching has been recognized as a consequence of a mechanical impingement between the humeral part and the lateral pillar of the scapula (2,3,15). Chemical osteolysis could be associated because of PE wear debris as a consequence of this mechanical impingement. Progression is controversial: some authors have reported that it appears during the first year and then stabilizes (14); whereas for others the frequency and the severity of notching have been observed to increase with time (11).

Clinical consequence

None has been reported regarding pain, Constant score and range of motion in most of the publications (9,10). Sirveaux et al were the only group to report a significant clinical influence (16).

How to avoid it

— Glenoid height: the base plate must be implanted low enough to have the sphere overhanging the inferior part of the scapula (13,14). Kelly et al proposed the 12mm rule with the Grammont type prosthesis in order to anticipate the obliquity of the scapula lateral pillar and the reaming (7).

— Tilt of the sphere: Inferior tilt of the sphere prevents contact between humerus and scapula in adduction. Some authors (13,14)) do not recognize a biomechanical or clinical influence of inferior tilt. Although inferior positioning is the most important factor, inferior tilting may help to prevent notching, moreover inferior tilting helps to prevent at any cost superior tilting which is always detrimental for notching and baseplate stability (12).

— Lateralization of the sphere: The lateralization may be part of the sphere design (5,8) or related to bone graft (1). It seems that the frequency of notching decreases with the amount of lateralization. Excessive lateralization may cause glenoid loosening due to the lever arm on the glenoid anchorage.

— Eccentric spheres are another option to lower the position of the sphere and clear the lateral scapular pillar (6).

— Decreased inclination angle: with 155° inclination of the humeral cup, the Grammont type design increases the potential for contact between the humerus and scapula. A lower inclination angle significantly decreases this risk (6,8) but increases the risk of prosthetic instability.

— Deltopectoral approach: This approach allows better exposure of the inferior part of the glenoid to satisfactorily position the baseplate. It is recognized that it is more difficult to implant the baseplate inferiorly with downward tilt using a superolateral approach (10,11).

— Preoperative superior erosion of the glenoid: The preoperative type of the glenoid erosion (types E2 and E3 according to Favard (4)) influences surgical positioning of the baseplate(12). Spontaneous upward rotation of the scapula in the coronal plane has also been recognized as a risk factor for notching because of the resulting inappropriate superior tilt of the glenoid side.

— Surgeon Experience: Rather than trying to solve the problem with one major effect (lateralization, inclination, glenoid height...) it is probably better (and less risky) to use a little bit of everything, i.e. 3 to 4 mm lateralization, 3 to 4 mm inferior overhanging, 10° inferior tilt, 145° inclination angle...

Conclusion

Although no definite relationship has been established between scapular notching and glenoid loosening in Reverse Shoulder Arthroplasty, and notching does not influence the long term follow-up (>10 years) of the reverse prosthesis, notching has been recognized as a disturbing and worrying radiological sign.